I believe the closest thing out of the box would be a Flex Relate field.
You can add one of these in studio to your Opportunities Module.
(Navigate to Admin>Studio>Opportunities>Fields, Add new field of Type “Flex Relate”, Add it to the Layouts)
However, some things to note are:
The Flex Relate field is not a full relate field, It only works one way.
So, if you select an Account on it, The Selection will not be shown on the Accounts -> Opportunities Subpanel.
Additionally, the Flex Relate by standard doesn’t let you select Users, But you can add the option fairly easily:
Create a file in the location: custom/include/language/ with the name “en_us.lang.php” (If this file already exists, you can simply add the following into it)
In this file, add the following:
'Accounts' => 'Account',
'Contacts' => 'Contact',
'Tasks' => 'Task',
'Opportunities' => 'Opportunity',
'Bugs' => 'Bug',
'Cases' => 'Case',
'Leads' => 'Lead',
'Project' => 'Project',
'ProjectTask' => 'Project Task',
'Prospects' => 'Target',
After this, Save your changes, and run a Quick Repair and Rebuild, and you should be able to select Users on Flex Relate field.